Adaptive Channel Prediction and Mitigating Interference in OFDM Systems

ABSTRACT

One embodiment provides an apparatus. The apparatus includes an optimization module configured to determine a guard interval remainder based, at least in part on a comparison of an allowable microreflection interference level and an actual microreflection interference level; and a windowing module configured to window an OFDM (orthogonal frequency division multiplexed) symbol utilizing the guard interval remainder. The apparatus may further include a channel estimator module configured to determine a predicted channel frequency response based, at least in part, on a probing symbol; and a pre-equalizer module configured to pre-equalize the OFDM symbol based, at least in part, on the predicted channel frequency response.

FIELD

The present disclosure relates to OFDM (orthogonal frequency divisionmultiplexing) systems, and, more particularly, to adaptive channelprediction and/or mitigating interference in OFDM systems.

BACKGROUND

In some communication systems, a receiver (and/or a transmitter) may becoupled to a plurality of transmitting (and/or receiving) devices via ashared communication network. The communication network may be shared,for example, by using OFDMA (orthogonal frequency division multipleaccess), a multi-user variant of OFDM (orthogonal frequency divisionmultiplexing). In OFDM, digital data, that may be encoded, is modulatedonto a plurality of orthogonal subcarrier frequencies (“subcarriers”).Modulation schemes may include, for example, QAM (quadrature amplitudemodulation) and/or PSK (phase shift keying). Subcarriers may furtherinclude so-called “pilots” that are subcarriers modulated with knowndata that may then be used for determining channel characteristics.

In OFDMA, each client device of a plurality of client devices mayutilize (e.g., may be assigned) a subset of the subcarriers. The clientdevice (transmitting or receiving) may then be configured to modulateuser data onto the subset of subcarriers and transform the modulatedsubcarriers (and pilots) into the time domain (using, e.g., an inversefast Fourier transform) to produce an OFDM symbol. The OFDM symbol maythen be converted from digital to analog, modulated onto an RF carrier(for example) to produce an analog signal and transmitted along achannel to a receiving device. The receiving device may be configured todemodulate the analog signal and digitize the demodulated analog signal(via, e.g., an analog to digital converter) to recover the OFDM symbol.The OFDM symbol may then be further processed to recover the user data.

The analog signal may be modified during its travel along the channeldue to channel characteristics that may change over time. Channelcharacteristics may include channel frequency response and/or channelimpulse response. In a multi-user system, channel characteristics mayvary across channels since a first communication channel that couples afirst client device to a head-end may be different from a secondcommunication channel that couples a second client device to thehead-end.

For example, in a broadband cable system, a cable modem terminationsystem (“head-end”) is configured to provide television, voice andnetwork access to a plurality of cable modems over a hybrid fibercoaxial cable network. A configuration of each channel coupling arespective cable modem to the head-end may vary across channels. Thus,channel characteristics may vary across channels. Channelcharacteristics may be estimated using periodic probing symbols. Eachchannel characteristic estimate may be applied to data-carrying OFDMsymbols until a next probing symbol is received and the channelcharacteristic estimate is updated. The probing symbols may betransmitted relatively far apart, e.g., at intervals ranging from aboutone second to about thirty seconds. Thus, several data-carrying OFDMsymbols may be transmitted (and received) between probing symbols.

The channel characteristics are assumed to be static, i.e., to remainconstant between probing symbols. However, channel characteristics mayactually change over the time interval between probing symbols. Further,small timing drifts may appear as changes in the channel frequencyresponse. Thus, the assumption that the channel characteristics arefixed between probing symbols may result in a loss of performance asequalization may be performed on data OFDM symbols using channelestimates that are no longer accurate.

Generally, in a receiving device successive received OFDM symbols areseparated (in time) by a guard interval. The guard interval isconfigured to accommodate echoes or micro-reflections from, e.g.,discontinuities in the channel. The guard interval may include a cyclicprefix used to facilitate performing a discrete Fourier transform (e.g.,a fast Fourier transform (FFT)) utilized in recovery of data from anOFDM symbol. Further, communication channels may be susceptible toin-band interference. Typically, in-band interference is not orthogonalto OFDM sub-carriers so that when the FFT is performed, a narrow bandinterferer may experience spectral widening due to the windowingoperation associated with the FFT. In other words, the window length isnot typically a whole number multiple of interferer frequency periods.Thus, an external interferer with a bandwidth of a few MHz (Megahertz)may spread to tens of MHz because of the windowing, thereby affecting anumber of sub-carriers and their associated data.

BRIEF DESCRIPTION OF DRAWINGS

Features and advantages of the claimed subject matter will be apparentfrom the following detailed description of embodiments consistenttherewith, which description should be considered with reference to theaccompanying drawings, wherein:

FIG. 1 illustrates a network system consistent with various embodimentsof the present disclosure; FIG. 2 illustrates an example of a sequenceof probing symbols consistent with various embodiment of the presentdisclosure;

FIG. 3 illustrates a communication system consistent with variousembodiments of the present disclosure;

FIG. 4 illustrates a graph of bit error rate (BER) versus signal tonoise ratio (SNR) for a simulated example communication systemconsistent with one embodiment of the present disclosure;

FIG. 5 illustrates an example of a portion of a sequence of receivedOFDM symbols and associated guard intervals consistent with oneembodiment of the present disclosure;

FIG. 6 illustrates an example of a portion of a channel impulse responseconsistent with one embodiment of the present disclosure;

FIG. 7 illustrates a graph of modulation error ratio (MER) versusfrequency for one example of a communications system consistent with thepresent disclosure;

FIG. 8 is a flowchart of operations for mitigating interferenceaccording to various embodiments of the present disclosure; and

FIG. 9 is a flowchart of adaptive channel prediction operationsaccording to various embodiments of the present disclosure.

Although the following Detailed Description will proceed with referencebeing made to illustrative embodiments, many alternatives,modifications, and variations thereof will be apparent to those skilledin the art.

DETAILED DESCRIPTION

Generally, this disclosure relates to communication systems (andmethods) configured to provide adaptive channel prediction and/ormitigate interference in OFDM systems. In an embodiment, adaptivechannel prediction (i.e., channel tracking) is configured to predictand/or interpolate a channel frequency response between channelfrequency response estimates that have been determined based on probingsymbols. The predicted channel frequency response may then be utilizedto pre-equalize and/or equalize OFDM symbols that include data. Theprediction and interpolation are configured to accommodate variation inthe channel characteristics between channel frequency response estimatesdetermined based on the probing symbols.

In another embodiment, a communication system and method consistent withthe present disclosure are configured to optimize sharing each guardinterval associated with a respective received OFDM symbol. A firstportion of the guard interval may be utilized for accommodatingmicro-reflections and a second portion of the guard interval (i.e.,guard interval remainder) may be utilized for windowing using, e.g., araised cosine window. A duration of the first portion may be determinedbased, at least in part, on a modulation scheme, e.g., 1024QAM (1024point quadrature amplitude modulation), a channel impulse response andsignal to noise ratio (SNR) specifications of the modulation scheme. Aduration of the guard interval remainder may then be determined based onthe size of the guard interval and the duration of the first portion.Windowing associated the with an FFT (fast Fourier transform) may thenbe performed at a receiving device with tapering configured to reducespectral spread associated with external interferers, as describedherein.

FIG. 1 illustrates a network system 100 consistent with variousembodiments of the present disclosure. Network system 100 generallyincludes a head-end 102, and at least one client device 106 a, . . . ,106 n. In some embodiments, head-end may correspond to a cable modemtermination system (CMTS) and each client device may correspond to acable modem. In some embodiments, the head-end may correspond to adigital subscriber line access multiplexer (DSLAM) and the client devicemay correspond to a DSL modem. The head-end 102 may typically beincluded in a services provider facility. One or more client device(s)106 a, . . . , 106 n may be located at respective customer locations.The head-end 102 is configured to communicate with each client device106 a, . . . , 106 n and each client device 106 a, . . . , 106 mconfigured to communicate with head-end 102 via communication network104. The communication network 104 may comprise, for example, opticalfiber, coaxial cable, copper twisted pair, copper twin-axial cable, etc.and/or a combination thereof, e.g., hybrid fiber coax (HFC). In someembodiments, the communication network 104 may include a plurality oflogical and/or physical channels (e.g., differential pair channels) thatprovide separate connections between, for example, a transmitter andreceiver of the head-end 102 and a receiver and transmitter,respectively, of a client device, e.g., client device 106 a.

The head-end 102 and each client device 106 a, . . . , 106 n maycommunicate with each other, via communication network 104, using a dataplus other service communications protocol, for example, a DOCSIS® (dataover cable service interface specification) communications protocol, aDSL (digital subscriber line) communications protocol, etc. The DOCSIS®communications protocol may comply or be compatible with the DOCSIS® 3.1specification published by CableLabs titled “DOCSIS® 3.1 Physical LayerInterface Specification, CM-SP-PHYv3.1-I01-131029”, released October2013 and/or later versions of this specification. The DSL protocol maycomply or be compatible with the ADSL (asymmetric digital subscriberline) Recommendation G.992.1 published by the TelecommunicationStandardization Sector of the International Telecommunication Union(ITU-T), titled “Asymmetric digital subscriber line (ADSL)transceivers”, published June 1999 and/or later versions of thisRecommendation, for example, ITU-T Recommendation G.992.3, titled“Asymmetric digital subscriber line transceivers 2 (ADSL2)”, publishedby the ITU-T July 2002. Of course, in other embodiments, the data plusother service communications protocol may include a custom and/orproprietary data plus other services communications protocol.

Head-end 102 and client device(s) 106 a, . . . , 106 n are configured tocommunicate using OFDMA (orthogonal frequency division multiple access).Head-end 102 may be configured to transmit one or more OFDM (orthogonalfrequency division multiplexing) symbols in an OFDMA symbol to clientdevice(s) 106 a, . . . , 106 n. Head-end 102 may be configured toreceive one or more OFDM symbols from client device(s) 106 a, . . . ,106 n. Thus, head-end 102 may correspond to a transmitting device or areceiving device. Each client device 106 a, . . . , 106 n is configuredto transmit or receive respective OFDM symbols to or from head-end 102.Thus, each client device 106 a, . . . , 106 n may be a transmittingdevice or a receiving device.

Each OFDM symbol includes a plurality of orthogonal subcarriers that mayinclude data subcarrier(s) modulated with data and pilot subcarriersconfigured to be used, e.g., for estimating channel characteristics.Head-end 102 may be configured to assign a respective subset ofavailable subcarriers of an OFDMA symbol to each client device 106 a, .. . , 106 n. Each client device 106 a, . . . , 106 n may then beconfigured to transmit and/or receive OFDM symbols that includerespective assigned subcarriers. Head-end 102 may then be configured totransmit and/or receive a plurality of OFDM symbols that collectivelyinclude all of the assigned subcarriers of the OFDMA symbol.

FIG. 2 illustrates an example sequence of OFDMA probing symbols 202 a,202 b, 202 c. In the example 200, the vertical axis corresponds tofrequency and the horizontal axis corresponds to time. Successiveprobing symbols are separated in time by time interval 204. For example,the time interval 204 duration may be on the order of ones or tens ofseconds. Each OFDMA probing symbol, e.g., probing symbol 202 a, mayinclude one or more probing OFDM symbols, e.g., OFDM probing symbols 206a, 208 a. The data subcarriers of OFDMA probing symbols 202 a, 202 b,202 c are configured to be modulated with probing information, e.g.,known data. The OFDMA probing symbols 202 a, 202 b, 202 c are configuredto be utilized for determining (estimating) channel characteristics, asdescribed herein.

The OFDMA probing symbols 202 a, 202 b, 202 c may be shared by aplurality of client devices, e.g., client device(s) 106 a, . . . , 106 nof FIG. 1. In an embodiment, each OFDMA probing symbol 202 a, 202 b, 202c may be utilized by a respective client device. For example, the firstOFDMA probing symbol 202 a may be assigned to a first client device,e.g., client device 106 a, the second OFDMA probing symbol 202 b may beassigned to a second client device, e.g. client device 106 b, andsubsequent OFDMA probing symbols may each be assigned to respectiveclient devices. The head-end 102 may then be configured to determine arespective frequency response corresponding to a full channel bandwidthfor each client device 106 a, . . . , 106 n based, at least in part, onthe respective OFDMA probing symbol.

In another embodiment, each OFDMA probing symbol 202 a, 202 b, 202 c maybe shared by a plurality of client devices, e.g., client devices 106 a,. . . , 106 n. OFDMA probing symbols 202 a, 202 b, 202 c may be shared,for example, when a respective client device is unable to transmit overthe full channel bandwidth, e.g., because of power limitations. Avariety of sharing schemes, including, e.g., interlaced sharing, may beutilized. For example, each client device 106 a, . . . , 106 n may beassigned a portion of each OFDMA probing symbol 202 a, 202 b, 202 c. Theportion assigned may change for each subsequent OFDMA probing symbol sothat over a sequence of OFDMA probing symbols a total of the portionsassigned to each client device 106 a, . . . , 106 n corresponds to acomplete OFDMA probing symbol. The head-end 102 is configured todetermine a respective portion of a channel frequency response for eachclient device 106 a, . . . , 106 n for each OFDMA probing symbol. Thehead-end 102 may thus determine a respective frequency response over afull channel bandwidth for each client device 106 a, . . . , 106 n bydetermining a respective frequency response for each portion of theOFDMA probing symbol for each client device 106 a, . . . , 106 n.

For example, OFDM probing symbol 206 a of first OFDMA probing symbol 202a may be assigned to first client device 106 a and OFDM probing symbol208 a of first OFDMA probing symbol 202 a may be assigned to secondclient device 106 b. OFDM probing symbol 208 b of second OFDMA probingsymbol 202 b may be assigned to first client device 106 a and OFDMprobing symbol 210 b of second OFDMA probing symbol 202 b may beassigned to second client device 106 b. OFDM probing symbol 210 c ofthird OFDMA probing symbol 202 c may be assigned to first client device106 a and OFDM probing symbol 212 c of third OFDMA probing symbol 202 cmay be assigned to second client device 106 b. The process may continuewith subsequent OFDMA probing symbols until the client devices 106 a,106 b have been assigned OFDM probing symbols covering the entire OFDMAprobing symbol, i.e., the full channel bandwidth. The plurality of OFDMprobing symbols for each client device 106 a, 106 b may then be utilizedto estimate channel characteristics (e.g., channel frequency response)for a first channel configured to couple, e.g., head-end 102 to firstclient device 106 a and a second channel configured to couple thehead-end 102 to second client device 106 b. Thus, a channelcharacteristic (e.g., channel frequency response) at a point in time(i.e., a snapshot) may be estimated based, at least in part, on one ormore OFDM probing symbols.

FIG. 3 illustrates a communication system 300 consistent with variousembodiments of the present disclosure. The communication system 300includes a head-end 302, a communication channel 304 and a client device306. Head-end 302 is one example of head-end 102 of FIG. 1 and clientdevice 306 is one example of client device(s) 106 a, . . . , 106 n ofFIG. 1. Communication channel 304 represents a portion of communicationnetwork 104, configured to couple head-end 302 to client device 306.Head-end 302 is configured to communicate (i.e., transmit and/or receiveinformation) with client device 306 via channel 304. Informationincludes commands, data, audio, video, voice, etc.

Head-end 302 and client device 306 each include a respective processor310, 330 configured to perform at least some of the operations of thehead-end 302 and client device 306, respective I/O circuitry 312, 332configured to couple head-end 302 and client device 306 to each othervia, e.g., channel 304 and respective memory 314, 334 configured tostore data, estimates, coefficients, etc., related to the respectiveoperations of head-end 302 and client device 306.

Each I/O circuitry 312, 332 includes respective PHY circuitry 320, 340that may each include transmit circuitry configured to transmit OFDMsymbols that may include data packets and/or frames via channel 304 andreceive circuitry configured to receive OFDM symbols that may includedata packets and/or frames via channel 304. Of course, PHY circuitry320, 340 may also include encoding/decoding circuitry configured toperform analog-to-digital and digital-to-analog conversion, encoding anddecoding of data, and recovery of received data.

Head-end 302 and client device 306 each include a respective FFT module316, 336 configured to determine an N-point discrete Fourier transform(e.g., a fast Fourier transform) and a respective IFFT module 318, 338configured to determine an N-point inverse discrete Fourier transform(e.g., an inverse fast Fourier transform). As is known, an FFT may beutilized to determine frequency spectrum (i.e., magnitude and phase) ofa finite duration of a discrete time signal and an inverse FFT may beutilized to produce a discrete time signal corresponding to a discretefrequency spectrum. For example, an impulse response of channel 304 maybe determined based, at least in part, on a frequency response ofchannel 304.

Channel 304 may be characterized by one or more channel characteristics.Such channel characteristics include channel impulse response andchannel frequency response. These channel characteristics are related tochannel physical properties such as length, discontinuities, branchesand/or terminations (that may result in reflection (i.e., echoes)related to a transmitted symbol) and channel media properties such asfinite bandwidth and/or nonlinear characteristics. Such channelproperties may distort (i.e., degrade) at least a portion of an OFDMsymbol and may therefore make recovering transmitted informationdifficult. Equalization may be used to compensate for such channelproperties and equalization parameters may be determined based, at leastin part, on an estimate of channel characteristics (e.g., channelimpulse response and/or channel frequency response). For example, thechannel frequency response may be determined based on one or moreprobing symbols, e.g., probing symbols 206 a, 206 b and 208 a, 208 b ofFIG. 2.

Equalization may be performed in a receiving device, e.g., head-end 302,on a received OFDM symbol and/or pre-equalization may be performed in atransmitting device, e.g., client device 306, prior to transmitting theOFDM symbol. Both equalization and pre-equalization parameters may bedetermined based, at least in part, on the estimates of channelcharacteristics (“channel estimates”). Channel characteristics may betime varying, thus, equalization and/or pre-equalization based onchannel estimates may be accurate for finite time periods. Channelestimates may be periodically updated (e.g., with a period correspondingto the time interval 304) and corresponding equalization and/orpre-equalization parameters may likewise be periodically updated.Equalization and/or pre-equalization performed during the time interval304 may initially correspond to actual channel characteristics but maydegrade between two successive probing symbols due to changing channelcharacteristics. Such degradation in equalization and/orpre-equalization may have limited effect on relatively small QAMconstellations (e.g., QAM64) but may have a relatively significanteffect on higher order QAM constellations (e.g., QAM1024, QAM4096).

In an embodiment, head-end 302 and client device 306 may be configuredto adaptively predict channel characteristics between probing symbolsand to thus improve performance of communication system 300 in thepresence of channel variation between probing symbols. Head-end 302 mayinclude a head-end channel estimator module 322 and client device 306may include a client device channel estimator module 342. Head-endchannel estimator module 322 is configured to request that client devicechannel estimator module 342 transmit a probing symbol at a timeinterval. The time interval may correspond to time interval 204 of FIG.2 and thus, may be on the order of ones to tens of seconds. The probingsymbol includes a known bit sequence that may then be utilized byhead-end channel estimator module 322 to estimate a channel frequencyresponse, H_(n)(k) where n corresponds to a probing symbol index, kcorresponds to a subcarrier index and H corresponds to frequencyresponse estimate determined based on a probing symbol. k may have arange of 0 to N−1 where N corresponds to a number of subcarriersassigned to the probing symbol. Thus, N may correspond to at least aportion of the subcarriers included in the OFDMA probing symbol, e.g.,probing symbol 202 a of FIG. 2. The channel estimates H_(n)(k), k=0, . .. , N−1, thus correspond to channel frequency response estimatesdetermined based, at least in part, on a probing symbol. The head-endchannel estimator module 322 may be configured to store the channelestimates 323 in memory 314. The head-end channel estimator module 322may then be configured to provide the estimated channel frequencyH_(n)(k) response to the client device channel estimator module 342,e.g., via I/O circuitry 312, channel 304 and I/O circuitry 332.

For example, the subcarriers of each OFDM probing symbol may bemodulated with a modulation pattern, e.g., binary phase-shift keying(BPSK), known to the head-end 302. The head-end 302 is configured toremove the known modulation pattern from a received OFDM probing symbol.The received subcarriers without the known modulation pattern may thencorrespond to the channel frequency response. For example, to BPSKmodulate a set of OFDM subcarriers, the client device 306 may multiplythe subcarriers with a sequence of +1 s and −1 s (i.e., plus ones andminus ones). The BPSK modulating sequence is known by the head-end 302.The head-end 302, e.g., channel estimator module 322, may then beconfigured to multiply the subcarriers of the received OFDM symbol withthe same sequence of +1 s and −1 s to remove the BPSK modulation toyield an estimate (i.e., a snapshot) of the channel frequency response.

Similarly, head-end 102 of FIG. 1 may be configured to request a probingsymbol from one or more of the plurality of client devices 106 a, . . ., 106 n, to estimate a respective channel frequency response for eachclient device 106 a, . . . , 106 n and to provide the respective channelestimate to the respective client device 106 a, . . . , 106 n. Thechannel estimates may differ due to differing channel properties betweeneach client device 106 a, . . . , 106 n and the head-end 102. Thus,head-end 102 and corresponding head-end 302 may not typically beconfigured to equalize OFDM symbols received from each client device 106a, . . . , 106 n. Each client device 106 a, . . . , 106 n may beconfigured to pre-equalize prior to transmitting respective OFDM symboldata to the head-end 102, as described herein.

Client device channel estimator module 342 may be configured to receivechannel estimate(s) H_(n)(k) for each probing symbol. Client devicechannel estimator module 342 is configured to store a history of channelestimates 343 in memory 334. For example, the history of channelestimates may include four channel estimates: H_(n-3)(k), H_(n-2)(k),H_(n-1)(k) and H_(n)(k), (k=0, . . . , N−1) where H_(n)(k) correspondsto the current channel estimate and H_(n-3)(k) corresponds to the oldestchannel estimate. In other embodiments, more or fewer channel estimatesmay be included in the history. Client device channel estimator module342 may then predict a next channel frequency response for eachsubcarrier index k based, at least in part, on the history of channelfrequency responses. The predicted next channel frequency response maybe determined as:

${{\hat{H}}_{n + 1}(k)} = {\sum\limits_{i = 0}^{3}\; {\alpha_{i}{H_{n - i}(k)}}}$

where Ĥ_(n+1)(k) is the predicted next channel frequency response forsubcarrier k, α_(i) is a linear predictor coefficient and H_(n-i)(k)corresponds to the i^(th) channel frequency response in the history ofchannel frequency responses. Client device channel estimator module 342may be configured to store the predicted next channel frequency response343 in memory 334. Client device channel estimator module 342 may beconfigured to maintain a history of predicted channel frequencyresponses, e.g. Ĥ_(n+1)(k) and Ĥ_(n) (k) for each subcarrier index k.

Client device 306 may include a pre-equalization module 346.Pre-equalization module 346 may be configured to interpolate betweenĤ_(n+1)(k) and Ĥ_(n)(k), i.e., between probing symbol n and probingsymbol n+1, to determine an estimated channel frequency response {tildeover (H)}_(m)(k) for each OFDM symbol between probing symbol n andprobing symbol n+1. For example, a number, e.g., M, OFDM (data) symbolsmay be transmitted between probing symbols, thus, m=0, 1, . . . , M−1.The estimated channel frequency response for each data OFDM symbol maybe determined as:

${{\overset{\sim}{H}}_{m}(k)} = {{\frac{M - m}{M}{{\hat{H}}_{n}(k)}} + {\frac{m}{M}{{{\hat{H}}_{n + 1}(k)}.}}}$

The equation for {tilde over (H)}_(m)(k) is configured to weight Ĥ_(n)(k) relatively more heavily for OFDM symbols relatively nearer thecurrent probing symbol n that corresponds to H_(n)(k) and to weightĤ_(n+1)(k) relatively more heavily for OFDM symbols relatively nearerthe next probing symbol n+1. In some embodiments, the predicted channelfrequency response Ĥ_(n)(k) may be utilized to determine {tilde over(H)}_(m)(k). In some embodiments, the channel frequency responseestimate H_(n)(k) provided by the head-end 302 may be utilized todetermine {tilde over (H)}_(m) (k). Utilizing Ĥ_(n) (k) may berelatively more desirable since H_(n)(k) may be noisy while Ĥ_(n)(k) mayinclude relatively less noise due to filtering.

The client device channel estimator module 342 is configured todetermine a prediction error, ε(k), in response to receiving the nextchannel estimate H_(n+1)(k) from head-end channel estimator module 322.The head-end channel estimator module 322 may be configured to determinethe next channel estimate H_(n+1)(k) in response to receiving probingsymbol n+1 from client device 306. The prediction error ε(k) may bedetermined as:

ε(k)=H _(n+1)(k)−Ĥ _(n+1)(k).

The client device channel estimator module 342 is configured to updatethe linear predictor coefficients α_(i) (i=0, 1, 2, 3) using anormalized least mean square (NLMS) technique. For example, the linearpredictor coefficients α_(i) may be determined as:

${\left. \alpha_{i}\Leftarrow{\alpha_{i} + {\mu \frac{1}{N}{\sum\limits_{k = 0}^{N - 1}\; {\frac{{H_{n - i}^{*}(k)}{ɛ(k)}}{\sum\limits_{l = 0}^{3}\; {{H_{n - l}(k)}}^{2}}\mspace{14mu} {for}\mspace{14mu} i}}}} \right. = 0},1,2,3.$

The linear predictor coefficients 345 may be stored in memory 334. Thus,the linear predictor coefficients may be updated based, at least inpart, on the history of channel estimates H_(n-i)(k), the error betweenthe next channel estimate H_(n+1)(k) and the predicted next channelfrequency response Ĥ_(n+1)(k) for N sub-carriers (i.e., k=0, 1, . . . ,N−1). The parameter μ is a weighting factor that affects convergence.For example, μ may be in the range zero to two. Selection of a value forthe parameter μ may be based, at least in part, on the dynamiccharacteristics of the channel relative to a measurement period. Forexample, if the channel is relatively slowly varying then the value of μmay be made relatively small. A relatively high initial value (e.g.,μ=0.25) may facilitate faster convergence of the channel frequencyresponse estimate. Once converged, for relatively slowly time varyingchannels, the value of μ may be made relatively small (e.g., μ=0.05).For relatively fast time varying channels the value of μ may remainrelatively high (e.g., μ=0.25).

In this embodiment, client device 306 is configured to performpredictive channel pre-equalization based, at least in part, on channelfrequency response data provided by head-end 302. Head-end 302 is notconfigured to perform equalization in this embodiment. Thus, channelchanges may be tracked with little or no latency.

In another embodiment, client device channel estimator module 342 may beconfigured to predict a time varying component of the channel frequencyresponse rather than the entire channel frequency response. In thisembodiment, a mean (i.e., average) channel frequency response, H _(n)(k)may be determined based, at least in part, on the channel frequencyresponse estimates 343 stored in memory 334. For example, the history ofchannel frequency response estimates may be summed and divided by thenumber of channel frequency response estimates in the history as:

${{\overset{\_}{H}}_{n}(k)} = \frac{\sum\limits_{i = 0}^{P - 1}\; {H_{n - i}(k)}}{P}$

For example, P may be equal to four. More or fewer channel frequencyresponse estimates may be used to determine the average channelfrequency response H _(n)(k). In another example, the mean value of thechannel frequency response may be determined using recursive filtering.A mean channel frequency response may be recursively updated for eachreceived estimate as:

H _(n)(k)∂(1−β) H _(n)(k)+βH _(n)(k)

The value of β is in the range of zero to one and may be set to arelatively small number, e.g., 1/16 (i.e., 0.0625).

The mean channel frequency response H _(n)(k) may then be subtractedfrom each channel estimate H_(n)(k) to yield the time varying component,e.g., H_(n)′ (k), of the channel frequency response. The error andlinear predictor coefficients may then be determined based, at least inpart, on the time varying component H_(n)′ (k) of the channel frequencyresponse rather than the channel frequency response H_(n)(k). Predictingbased on the time varying component of the channel frequency responsemay reduce a dynamic range associated with components involved in theprediction and may result in a relatively better channel estimate.Reducing the dynamic range may facilitate fixed point arithmetic (i.e.,may reduce the number of bits needed to represent, e.g., a channelfrequency response) associated with predicting channel frequencyresponse (e.g., error and linear predictor coefficients) and thus mayresult in a more efficient implementation.

In another embodiment, client device pre-equalization module 346 may beconfigured to pre-equalize OFDM symbols using the current channelestimate H_(n)(k) from head-end channel estimator module 322 rather than{tilde over (H)}_(m)(k). In this embodiment, the head-end 302, e.g.,head-end channel estimator module 322, may be configured to predict aresidual component (i.e., time varying component) of the channelfrequency response. For example, a history of channel frequency responseestimates 323 may be stored in memory 314. The linear predictorcoefficients may be determined based, at least in part, on the residualcomponents. Head-end equalizer module 326 may then be configured toperform equalization of received pre-equalized OFDM symbols based, atleast in part, on predicted residual channel estimates.

In another embodiment, client device pre-equalization module 346 may beconfigured to pre-equalize the probing symbol, prior to transmitting theprobing symbol to the head-end 302. The client device pre-equalizationmodule 346 may be configured to pre-equalize the probing symbol based onthe latest channel estimate (e.g., H_(n)(k)) received from the head-end302. The head-end channel estimator module 322 may then be configured todetermine a channel frequency response (i.e., channel estimateH_(n+1)(k)) based, at least in part on the received pre-equalizedprobing symbol. If the channel frequency response hasn't changed betweenH_(n)(k) and H_(n+1)(k), then the received pre-equalized probing symbolcorresponds to the probing symbol. If the channel frequency response haschanged, then the channel estimate determined by the head-end channelestimator module 322 may correspond to the time varying component.

Continuing with this embodiment, client device pre-equalization module346 is configured to pre-equalize the n+1 probing symbol based on thecurrent channel estimate H_(n)(k) and to transmit this pre-equalizedprobing symbol to head-end 302. Client device pre-equalization module346 is also configured to pre-equalize data OFDM symbols. The head-endchannel estimator module 322 is configured to determine a channelestimate E_(n+1)(k) (k=0, . . . , N−1) from the received pre-equalizedprobing symbol. Thus, E_(n+1) (k) represents a channel frequencyresponse estimate based, at least in part, on a pre-equalized probingsymbol. If there have been no channel frequency response variationssince determining the prior channel estimate H_(n)(k), then the head-endchannel estimate E_(n+1)(k) should be unity (i.e., one) for allsubcarriers in the probing symbol, i.e., all values of k. If the channelfrequency response has changed then the E_(n+1)(k) represents the changein the channel frequency response. The head-end channel estimator module326 may then configured to provide the channel estimate E_(n+1)(k) toclient device 306 for pre-equalization purposes. Client device channelestimator module 342 is then configured to multiply the priorpre-equalization frequency response by the channel estimate to get a newchannel frequency response for pre-equalization. The new channelfrequency response may be determined as:

H _(n)(k)=H _(n-1)(k)*E _(n)(k)

where E_(n)(k) is the current channel estimate from the head-end,H_(n-1)(k) is the prior pre-equalization frequency response and H_(n)(k)is the new channel frequency response for pre-equalization. Thus,H_(n)(k) corresponds to the prior pre-equalization frequency responseupdated with the head-end new estimate that includes channel variation(if any) since the prior estimate. The client device 306 may then usethis channel estimate in the prediction-based algorithm, as describedherein, for pre-equalization.

The head-end 302 may be configured to retain a history of a number(e.g., four) successive estimates, e.g., E_(n-3)(k), E_(n-2)(k),E_(n-1)(k) and E_(n)(k). These estimates may be stored, for example, inmemory 314 (channel estimates 323). Head-end channel estimator module322 may then be configured to predict a next estimate Ê_(n+1)(k) as:

${{\hat{E}}_{n + 1}(k)} = {\sum\limits_{i = 0}^{3}\; {\alpha_{i}{{E_{n - i}(k)}.}}}$

Head-end equalizer module 326 is configured to linearly interpolatebetween Ê_(n) (k) and Ê_(n+1)(k), similar to pre-equalization module346, as described herein. The head-end estimate for each OFDM symbolbetween probing symbols may then be determined as:

${{{\overset{\sim}{E}}_{m}(k)} = {{{\frac{L - m}{L}{{\hat{E}}_{n}(k)}} + {\frac{m}{L}{{\hat{E}}_{n + 1}(k)}\mspace{14mu} {for}\mspace{14mu} m}} = 0}},1,\ldots \mspace{14mu},{L - 1.}$

Head-end equalizer module 326 may then utilize {tilde over (E)}_(m) (k)to equalize the m^(th) OFDM symbol between two successive probingsymbols n and n+1. Thus, the m^(th) OFDM symbol may be pre-equalized bythe channel frequency response H_(n)(k) at client device 306 (by, e.g.,client device pre-equalization module 346) and equalized by thepredicted channel frequency response {tilde over (E)}_(m)(k) at thehead-end 302 (by, e.g., head-end equalizer module 326). When the nextprobing symbol is received, the head-end channel estimator module 322 isconfigured to determine a prediction error as:

ε(k)=E _(n+1)(k)−Ê _(n+1)(k).

The head-end channel estimator module 322 is configured to update thelinear predictor coefficients α_(i) (i=0, 1, 2, 3) using a normalizedleast mean square (NLMS) technique, as described herein. The linearpredictor coefficients 325 may be stored in memory 314. Thus, in thisembodiment, the client device 306 is configured to performnon-predictive pre-equalization and the head-end 302 is configured toperform predictive residual equalization.

FIG. 4 illustrates a graph 400 of bit error rate (BER) versus signal tonoise ratio (SNR) for a simulated example communication systemconsistent with one embodiment of the present disclosure. In thisexample, a client device is configured to perform non-predictivepre-equalization and the head-end is configured to perform predictiveresidual equalization. In this example, a client device withpre-equalization was configured to transmit a 16k (i.e., 16,384 point)FFT OFDM (80 μs (microsecond) symbol size) modulated with a QAMconstellation 1024. The channel was simulated to include both multi-path(i.e. delayed symbols) and a Doppler effect. Multi-path included an echowith four paths: −10 dB (decibel) at 0.5 μs, −15 dB at 1.0 μs, −20 dB at1.5 μs and −30 dB at 4.5 μs. To model relatively slow channel changes,additional paths have been superimposed (at these same delays) with arelative strength of −20 dB (with respect to the path at that delay) and5.0 Hz of pure Doppler. The head-end was modeled as an OFDM receiverwith a channel equalizer and an LPDC (low density parity check) decoderwith block size 16,200 bits and rate 8/9.

Graph 400 illustrates the performance of the entire communicationsystem, e.g., communication system 300 without external interference.The vertical axis is the bit error rate (BER) at the output of the LDPCdecoder and the horizontal axis is the signal to noise ratio (SNR) indB. Line 402 illustrates additive white Gaussian noise (AWGN)performance in the absence of both multipath (e.g., echo) and Dopplereffects. Line 404 illustrates performance when predictive equalizationis not used, pre-equalization was used in the transmitting device (e.g.,H_(n)(k)) but without predictive channel equalization in the receivingdevice (head-end). The illustrated performance loss may be due to notaccounting for the relatively slow channel frequency response variationsbetween probing symbols used to estimate the channel frequency response.In this example, the probing symbols were spaced every 45 OFDM symbols.Line 406 illustrates the combination of pre-equalization in thetransmitting device (i.e., client device) and predictive channelestimation and equalization in the receiving device (i.e., head-end).Line 406 illustrates superior performance as compared to line 404 and isrelatively closer to line 402. It is expected that performance gainswill be higher for higher order constellations and/or a greater numberof time varying channels.

Thus, a communication system consistent with the present disclosure maybe configured to adaptively predict channel characteristics, i.e.,channel frequency response, based, at least in part, on a plurality ofprior channel frequency response estimates and a plurality of predictioncoefficients. The channel frequency response estimates may be determinedbased on probing symbols and the prediction coefficients may bedetermined based, at least in part, on a difference between an estimatedchannel frequency response and a predicted channel frequency responseusing a normalized least mean square technique. Pre-equalization and/orequalization may then be performed on OFDM symbols transmitted andreceived in a time interval between the probing symbols. In someembodiments, the pre-equalization and/or equalization may be performedbased on the time varying component of the channel frequency responseconfigured to provide improved sensitivity and improved accuracy. Thus,variation in channel frequency response between probing symbols may beaccommodated.

Turning again to FIG. 3, in another embodiment, communication system 300may be configured to reduce spectral spread of an external interferencein an OFDM symbol while mitigating (i.e., limiting) microreflectioninterference (e.g., intersymbol interference (ISI) and intercarrierinterference (ICI)), as described herein. External interference, e.g.,external interferer 308, may be introduced into channel 304 and maycombine with an ODFM symbol as it travels from a transmitting device toa receiving device. The head-end 302 may be the transmitting device andthe client device 306 may be the receiving device or the client device306 may be the transmitting device and the head-end 302 may be thereceiving device. The external interferer 308 may be narrow band with acarrier frequency that is in-band relative to the OFDM symbol. Forexample, the external interferer 308 may correspond to a terrestrialelectromagnetic signal associated with cellular telephones and the OFDMsymbol may correspond to wired electromagnetic signal.

Microreflection interference includes intersymbol interference andintercarrier interference. Intersymbol interference occurs whenreflections (i.e., echoes) of a previous OFDM symbol arrive at thereceiving device during reception of a current OFDM symbol. Intercarrierinterference accompanies intersymbol interference and occurs as a resultof a reflected version of a OFDM symbol being truncated by the OFDM FFTwindow. Intersymbol and intercarrier interference may be mitigated byadding a guard interval between successive OFDM symbols.

FIG. 5 illustrates an example 500 of a portion of a sequence of receivedOFDM symbols and associated guard intervals. In this example 500, thehorizontal axis corresponds to time. The example 500 includes a usefulOFDM symbol 502 preceded by an associated guard interval 504. Guardinterval 504 may be preceded on the left by another useful OFDM symbolpreceded by another guard interval, etc., and OFDM symbol 502 may befollowed on the right by another guard interval followed by anotheruseful OFDM symbol, etc. A duration of the guard interval 504 is T_(G)and a duration of the useful OFDM symbol 502 (i.e., symbol period) isT_(U). OFDM symbol period T_(S) corresponds to the duration of the guardinterval T_(G) plus the useful symbol period T_(U).

As part of data recovery at the receiving device, a window, e.g., window506, may be applied to the OFDM symbol 502 that is configured to providea time-limited input to an FFT. A “boxcar”—shaped window is desirablebecause it is flat over the window width and, thus, does not changerelative amplitudes of the frequency components in the OFDM signal. Aboxcar-shaped window that has relatively sharp edges may result inspectral dispersion of the subcarriers because the OFDM symbol may havea non-zero amplitude subcarriers at the window edges (i.e., Fouriertransform of pulse is a sinc function). The orthogonality characteristicof OFDM subcarriers is configured so that such spectral dispersion ofthe subcarriers does not cause interference when recovering data. Theboxcar window in the receiving device applied to the externalinterference results in spectral dispersion so that a relatively narrowband interferer (e.g., 2 or 3 MHz) may affect tens of MHz of OFDMsubcarriers. Since the external interference is typically not orthogonalto the OFDM subcarriers, such spectral dispersion can negatively affectdata recovery at the receiving device. Such dispersion effects may bemitigated by utilizing a window with tapered (e.g., raised cosinewindow) edges, e.g., window 506 and tapered edges 508, 510. The taperededges 508, 510 of the window are configured to ensure that an amplitudeof the OFDM symbol (and the external interferer) is at or near zero atthe start and the end of the window. Elimination of the sharp edgesreduces the spectral dispersion.

A cyclic prefix may be added to the start of a useful OFDM symbol by atransmitter. The cyclic prefix is simply a tail portion of the usefulOFDM symbol that corresponds to a fraction (e.g., ⅛) of the useful OFDMsymbol period, T. The cyclic prefix is configured to occupy the guardinterval 504, thus, the terms “guard interval” and “cyclic prefix” maybe used interchangeably. The cyclic prefix is configured to facilitatetriggering the FFT at the receiver. When the window 506 with taperededges 508, 510 is applied to the OFDM symbol 502, in order to operate onthe full useful ODFM symbol period, T_(U), the taper 508 at the start ofthe window 506 may occupy a portion 514 of the guard interval 504 whilethe taper 510 at the end of the window 506 occupies a portion 516 of theOFDM symbol 502 period T. An effect of taper 510 is to reduce theamplitude of the useful symbol 502 in the taper region 516. Because ofthe cyclic nature of the FFT, an effective boxcar window may be producedby, e.g., overlapping and adding the FFT results corresponding to theinitial taper 508 (i.e., region 514) with the FFT results correspondingto the end taper 510 (i.e., region 516). In other words, for the OFDMsymbol, the tapered region 514 (i.e., the cyclic prefix occupying region514) is equivalent to tapered region 516 (i.e., the portion of the OFDMsymbol occupying region 516). This is because of the cyclic prefixproperty, i.e., region 514 of the guard interval 504 includes areplication of region 516 of the OFDM symbol. Therefore, when thereplication of the portion of the OFDM symbol included in the taperedregion 514 is added to the portion of the OFDM symbol in the taperedregion 516, the tapering cancels, and as a result a boxcar effectivewindow is applied to the useful OFDM symbol 502. Interference, e.g.,interference 308 of FIG. 3, does not have the cyclic prefix property andhence is windowed (including taper) within useful symbol duration 502.Further, the window length for interference is the full window length(i.e., T_(U) plus (T_(G)−{tilde over (T)}_(G))) which may be longer thanT_(U), but the overlap-add operation fits the entire window into T_(U)interval.

To maximize the effects of the taper 508, 510 (i.e., minimize the spreadof the external interferor), it is desirable to utilize as much of theguard interval 504 (i.e., as much of T_(G)) as possible for tapering.However, a purpose of the guard interval 504 is to reduce the effects ofmicroreflection interference on the received OFDM symbol 502. If theguard interval 504 is fully utilized by the window 506 (including taper508) then contributions from echoes may produce microreflectioninterference (i.e., ISI and ICI). To minimize the microreflectioninterference, it is desirable to utilize as much of the guard interval504 (i.e., as much of T_(G)) as possible. In other words, to minimizemicroreflection interference, it is desirable for trigger point P to berelatively closer to useful OFDM symbol 502. Thus, utilizing the guardinterval 504 to minimize microreflection interference conflicts withutilizing the guard interval 504 to minimize the effects of the externalinterferer 308 on the OFDM symbol processing.

A system and method consistent with the present disclosure areconfigured to optimize mitigating the micro-reflection interference(i.e., reducing or limiting a microreflection interference level) andmitigating the effects of the external interferer, utilizing the guardinterval 504 (i.e., T_(G)). Referring again to FIG. 5, for example, if adelay of the furthest microreflection is less than {tilde over(T)}′_(G), i.e., microreflection(s) may appear in the first region 512of the guard interval, then a boxcar FFT may be triggered at any pointin the second region 514 of the guard interval without introducingintersymbol or intercarrier interference due to microreflections. Thesecond region 514, of duration (T_(G)−{tilde over (T)}′_(G)), may thenbe used for tapering as shown in FIG. 5. In another example, the boxcarFFT may be triggered in a third region 518 that corresponds to a portionof the first region 512. Triggering the boxcar FFT in the third region518 may introduce some intersymbol and intercarrier interference, butmay allow a larger region 520 (i.e., of duration T_(G)−{tilde over(T)}_(G), where {tilde over (T)}_(G)<{tilde over (T)}′_(G)) to beutilized for tapering. Utilizing the relatively larger region 520 (i.e.,a guard interval remainder) may then mitigate and/or reduce further theeffects of the external interferer when compared to the relativelysmaller second region 514. The channel impulse response may bedetermined at the receiving device and, knowing the OFDM modulation, itis possible to identify a point within the guard interval 504 to triggerthe boxcar FFT configured to provide a tolerable level of intersymboland intercarrier interference. The remaining region (i.e., the guardinterval remainder of duration T_(G)−{tilde over (T)}_(G)) within thecyclic prefix guard interval 504 may then be used for tapering.

Turning again to FIG. 3, head-end 302 and client device 306 may includerespective optimization modules 324, 344 and respective windowingmodules 328, 348. Respective memory 314, 334 are each configured tostore respective reflection data 327, 347 and respective window data329, 349. Optimization modules 324, 344 are configured to determine aguard interval remainder based, at least in part, on a comparison of anallowable microreflection interference level and an actualmicroreflection interference level. A first portion of the guardinterval may then be utilized to mitigate interference frommicroreflections and the guard interval remainder may be utilized forwindow taper (i.e., to mitigate spectral spread from externalinterferors). In the following discussion, client device 306 correspondsto the receiving device and head-end 302 corresponds to the transmittingdevice. Of course, a similar description applies when head-end 302 isthe receiving device and client device 306 is the transmitting devicewith client device optimization module 344 replaced by head-endoptimization module 324, client device windowing module 348 replaced byhead-end windowing module 328, etc.

Client device channel estimator module 342 is configured to determine afrequency response of channel 304. For example, client device channelestimator module 342 may be configured to estimate the channel frequencyresponse based, at least in part, on one or more pilot signals includedin a received OFDM symbol. The channel frequency response may typicallybe estimated to facilitate equalization, as described herein. Thus, insome embodiments, the channel frequency response may be available to theclient device optimization module 344 initially. In other embodiments,the client device channel estimator module 342 may be configured todetermine the channel frequency response estimate in response to arequest from the client device optimization module 344. The IFFT module338 may then be configured to determine a channel impulse response forchannel 304, the channel impulse response based, at least in part, onthe estimated channel frequency response. In some embodiments, aplurality of IFFTs may be performed and the magnitudes of themicroreflections in the resulting respective channel impulse responsesmay be squared and averaged to reduce noise effects.

FIG. 6 illustrates an example 600 of a portion of a channel impulseresponse (i.e., echo profile) where the horizontal axis corresponds totime. The echo profile 600 represents relative power levels and delaysassociated with the transmitted symbol and its associatedmicroreflections. The example 600 includes a main symbol 602 (i.e.,transmitted symbol) and a plurality of micro-reflections (i.e., echoes)604, 606, 608, 610. The respective amplitude (representing power level)of each echo 604, 606, 608, 610 decreases as time increases, in thisexample 600. The respective strengths (i.e., power level) and delays ofeach micro-reflection 604, 606, 608, 610 may be determined from theIFFT. A duration T_(G) corresponds to the duration T_(G) of the guardinterval 504 of FIG. 5 and a duration {tilde over (T)}_(G) correspondsto the adjusted guard interval. Thus, echo 608 occurs within the guardinterval T_(G) but outside the adjusted guard interval {tilde over(T)}_(G). Echo 610 is outside of the guard interval, T_(G), and thus,will give rise to ISI and ICI (i.e., microreflection interference) thatcannot be mitigated. However, the amount of microreflection interferencedue to echo 610 may be relatively small and thus, may be less than anallowable microreflection interference level.

In some embodiments, client device optimization module 344 may beconfigured to apply a threshold to the echo profile 600. The thresholdis configured to exclude relatively small echoes that may not contributesignificantly to the microreflection interference level. In other words,such relatively small microreflections may appear in the guard intervalremainder 520, or in the time interval T_(U) of FIG. 5. The optimizationmodule 344 may be configured to determine an echo power E_(τ) and anecho delay τ, i.e., a time interval between the start of the guardinterval and the location of the associated echo for each echo 604, 606,608, 610. For example, T in echo profile 600 corresponds to the echodelay of echo 610.

The client device optimization module 324 may then be configured todetermine an actual microreflection interference level, i.e., theintersymbol and intercarrier interference due to microreflection, forone or more adjusted guard intervals {tilde over (T)}_(G). Initially,{tilde over (T)}_(G) may be set equal to the guard interval T_(G) (i.e.,zero guard interval remainder). Thus, a minimum actual microreflectioninterference level may be determined. The client device optimizationmodule 344 may then be configured to compare the minimum actualmicroreflection interference level to an allowable microreflectioninterference level. If the minimum actual microreflection interferencelevel is less than the allowable microreflection interference level,then at least some of the guard interval T_(G) may be available forwindowing. The client device optimization module 344 may then beconfigured to decrease {tilde over (T)}_(G), determine the actualmicroreflection interference level and compare the minimum actualmicroreflection interference level to the allowable microreflectioninterference level. The process may repeat as long as the actualmicroreflection interference level is less than the allowablemicroreflection interference level. Eventually, the actualmicroreflection interference level may be greater than the allowablemicroreflection interference level. The adjusted guard interval durationT_(G) may then be increased until the actual microreflectioninterference level is less than or equal to the allowablemicroreflection interference level. The remainder guard interval maythen be determined as T_(G)−{tilde over (T)}_(G).

For example, the client device optimization module 344 may be configuredto determine an estimate of a respective contribution to microreflectioninterference for each echo that is outside the adjusted guard interval{tilde over (T)}_(G) as:

${{ICI} \approx {ISI}} = {\left( \frac{\tau - {\overset{\sim}{T}}_{G}}{T_{U}} \right)E_{\tau}}$

where τ is echo delay, {tilde over (T)}_(G) is the adjusted guardinterval duration, T_(U) is OFDM useful symbol duration and E_(τ) isecho power. Thus, microreflection interference may result from acomponent of each echo (τ−{tilde over (T)}_(G)) that is outside theadjusted guard interval {tilde over (T)}_(G). In this estimate, thecontribution from inter-symbol interference is assumed to beapproximately equal to the associated contribution from inter-carrierinterference, thus the ISI and ICI (i.e., the actual microreflectioninterference level) for each echo is approximately

$2*\left( \frac{\tau - {\overset{\sim}{T}}_{G}}{T_{U}} \right){E_{\tau}.}$

The contributions from each of the echoes outside the adjusted guardinterval, e.g., echoes 608, 610 may be determined and summed to yieldthe actual microreflection interference level.

An allowable micro-reflection interference level (i.e., combination ofISI+ICI for the echoes) may be related to modulation technique, e.g., aparticular QAM constellation. For example, each QAM constellation mayhave an associated maximum allowable micro-reflection interferencelevel. Thus, client device optimization module 344 may be configured todetermine the associated allowable micro-reflection level based on theparticular QAM constellation. For example, reflection data 347 mayinclude a look-up table configured to relate the allowablemicroreflection level to QAM constellation. In some embodiments, theallowable microreflection level may be provided as a signal to noiseratio (SNR). The optimization module 344 may be configured to determinean allowable microreflection level based, at least in part, on theprovided SNR.

Client device optimization module 344 is configured to adjust {tildeover (T)}_(G), determine the total ISI and ICI from echoes outside{tilde over (T)}_(G) and compare the total ISI and ICI (i.e., the actualmicroreflection interference level) to the allowable micro-reflectioninterference level for the particular modulation technique. In someembodiments, the optimum {tilde over (T)}_(G) may be determined bymonitoring the resulting SNR, determined based, at least in part, onpilot sub-carrier(s). For example, the optimum {tilde over (T)}_(G) maycorrespond to the minimum {tilde over (T)}_(G) that provides an actualmicroreflection interference level that is below the allowablemicro-reflection interference level. The remainder of the guard interval(i.e., T_(G)−{tilde over (T)}_(G)) may then be utilized for windowing.

Client device windowing module 348 may be configured to generate windowdata 349 based, at least in part, on the guard interval remainder.Client device windowing module 348 may then be configured to window eachuseful OFDM symbol, e.g., useful OFDM symbol 502, prior to client deviceFFT module 336 performing an FFT. Thus, interference effects from theexternal interferer 308 may be mitigated while preserving the SNRassociated with microreflections.

For example, referring to FIG. 5 and FIG. 6 for illustrative purposes,assume an OFDM FFT size of 4096 covering a 204.8 MHz bandwidth with asymbol duration, T_(S), of 20 μs (microsecond) and a maximum allowablemicro-reflection interference level of −40.0 dB. Assume also a cyclicprefix (i.e., guard interval T_(G)) of 2.5 μs and one echo of strength−30 dB (decibels) corresponding to E_(τ) with a delay τ of 2.8 μs. Ifthe entire guard interval T_(G) can be used for cyclic prefix and FFTtriggering, then the actual microreflection interference level can becalculated as approximately:

${E_{\tau}\left( {{in}\mspace{14mu} {dB}} \right)} + {10*{\log_{10}\left( {2*\frac{\tau - T_{G}}{T_{S}}} \right)}{in}\mspace{14mu} {dB}}$${{or} - 30 + {10*{\log_{10}\left( {2*\frac{2.8 - 2.5}{20}} \right)}}} = {{- 45.3}\mspace{14mu} {dB}}$

Since −45.3 dB is less than −40.0 dB (i.e., allowable micro-reflectioninterference level), the adjusted cyclic prefix interval {tilde over(T)}_(G) may be less than T_(G). For example, substituting {tilde over(T)}_(G)=1.8 μs for T_(G) yields an actual microreflection interferencelevel of −40.0 dB which meets the allowable micro-reflectioninterference level criterion. Thus, 0.7 μs (i.e., 2.5−1.8) may now beavailable for windowing. In operation, the trigger point for applyingthe window corresponds to 1.8 μs from the start of the guard intervalT_(G). The window may be applied by, e.g., client device windowingmodule 248, and an overlap-add operation may be performed. The FFT maythen be triggered at the end of the guard interval T_(G), correspondingto an optimal trigger point without windowing. Thus, the full length ofthe window (T_(U)+(T_(G)−{tilde over (T)}_(G))) may be free of ISI dueto the main path and ICI caused by the main path due to the windowingmay be avoided, i.e., perfect reconstruction. The final trigger pointfor the FFT may correspond to an optimal trigger point for the no-windowcase and the no-window case resulted in an actual microreflectioninterference level of about −45 dB. Thus, minor changes in ICI and ISIlevels, in the windowed case, for echoes outside the guard interval maynot result in exceeding the −40 dB target. In other words, perfectreconstruction may be achieved with reference to the main path but notwith reference to a relatively weak reflection with delay 2.8 μs. Therelatively weak reflection may result in a microreflection interferencelevel of −40 dB. Since an actual microreflection interference level of−40 dB does not exceed the allowable micro-reflection interferencelevel, this relatively weak reflection may be deemed tolerable.

FIG. 7 illustrates a graph 700 of modulation error ratio (MER) versusfrequency for one example of a communications system consistent with thepresent disclosure, in the presence of an external interferer (e.g., LTEwireless signal). This example corresponds to an OFDM FFT size of 4096covering a 204.8 MHz bandwidth with a symbol duration, T_(S), of 20 μs,a cyclic prefix (i.e., guard interval T_(G)) of 2.5 μs and {tilde over(T)}_(G)=1.8 μs. Thus, a guard interval remainder of 0.7 μs (i.e.,2.5−1.8) of the guard interval T_(G) was utilized for time domainwindowing, as described herein. Plot 702 illustrates MER for OFDMsymbols without time domain windowing. A relatively narrow bandinterferer results in significant spectral dispersion over the bandwidthof the OFDM system. Plot 704 illustrates MER for OFDM symbols with timedomain windowing. The duration of the roll-off of the window edgecorresponds to 0.7 μs divided by 20 μs, i.e., 3.5%. The spectraldispersion of plot 704 is significantly less than the spectraldispersion of plot 702. Thus, time domain windowing utilizing a portionof a guard interval (i.e., guard interval remainder) may reduce spectraldispersion of external interferers.

Thus, a portion of a guard interval (i.e., guard interval remainder) maybe utilized for time domain windowing to reduce spectral dispersion ofan external interferer. The guard interval remainder may be determinedbased, at least in part, on allowable microreflection interference levelassociated with a modulation technique for an OFDM symbol. Themicroreflection interference level may be limited and the spectraldispersion may then be mitigated by sharing the guard interval.

FIG. 8 is a flowchart 800 of operations for mitigating interferenceaccording to various embodiments of the present disclosure. Inparticular, the flowchart 800 illustrates optimizing sharing a guardinterval to provide windowing for mitigating spectral spread fromexternal interferors and limiting a microreflection interference level.Operations of this embodiment may be performed by a head-end and/or aclient device. Operations of this embodiment include determining achannel frequency response 802. Operation 804 includes determining achannel impulse response. The channel impulse response may be determinedby performing an IFFT on the channel frequency response. An actualmicroreflection interference level may be determined at operation 806.The actual microreflection interference level is configured to includecontributions from intersymbol interference and contributions fromintercarrier interference. An allowable microreflection interferencelevel may be determined at operation 808. The allowable microreflectioninterference level may depend on modulation technique. Whether theactual microreflection interference level is less than the allowablemicroreflection interference level may be determined at operation 810.If the actual microreflection interference level is less than theallowable microreflection interference level, then an adjusted guardinterval may be reduced at operation 812 and program flow may return tooperation 806. If the actual microreflection interference level is notless than the allowable microreflection interference level, then theadjusted guard interval may be increased until the actualmicroreflection interference level is less than or equal to theallowable microreflection interference level at operation 814. Operation816 may include determining a guard interval remainder. For example, theguard interval remainder may correspond to a difference between theguard interval duration and the adjusted guard interval. Operations 818may include windowing an OFDM symbol using the guard interval remainder.Program flow may then return at operation 820.

The operations of flowchart 800 are configured to determine a guardinterval remainder based, at least in part, on a comparison of anallowable microreflection interference level and an actualmicroreflection interference level. The operations of flowchart 800 arefurther configured to window and OFDM symbol utilizing the guardinterval remainder.

FIG. 9 is a flowchart 900 of adaptive channel prediction operationsaccording to various embodiments of the present disclosure. Inparticular, the flowchart 900 illustrates one example embodiment ofoperations of a communication system configured to pre-equalize and/orequalize an OFDM symbol based, at least in part, on a predicted channelcharacteristic, e.g., channel frequency response. Operations of thisembodiment may be performed by a head-end and/or a client device.Operations of this embodiment include transmitting a probing symbol 902.For example, a client device may be configured to transmit a probingsymbol in response to a request from a head-end. In some embodiments,the probing symbol may be pre-equalized prior to transmitting. A channelfrequency response estimate may be determined based on the probingsymbol at operation 904. Operation 906 may include predicting a nextchannel frequency response based, at least in part, on the channelfrequency response estimate and a linear predictor coefficient. An OFDMsignal may be pre-equalized and/or equalized based, at least in part, onthe predicted channel frequency response at operation 908. The linearpredictor coefficient may be updated based, at least in part, on thepredicted next channel frequency response and a next channel frequencyresponse estimate at operation 910. In some embodiments, at operation912 a next probing symbol may be pre-equalized prior to transmitting.Program flow may then proceed to operation 902.

The operations of flowchart 900 are configured to determine a predictedchannel characteristic based, at least in part, on a probing symbol andto equalize and/or pre-equalize an OFDM symbol based, at least in part,on the predicted channel characteristic.

While the flowcharts of FIGS. 8 and 9 illustrate operations according tovarious embodiments, it is to be understood that not all of theoperations depicted in FIGS. 8 and/or 9 are necessary for otherembodiments. In addition, it is fully contemplated herein that in otherembodiments of the present disclosure, the operations depicted in FIGS.8 and/or 9, and/or other operations described herein may be combined ina manner not specifically shown in any of the drawings, and suchembodiments may include less or more operations than are illustrated inFIGS. 8 and/or 9. Thus, claims directed to features and/or operationsthat are not exactly shown in one drawing are deemed within the scopeand content of the present disclosure.

The foregoing provides example system architectures and methodologies,however, modifications to the present disclosure are possible. Forexample, head-end 102 and/or client device(s) 106 a, . . . , 106 n mayalso include a host processor, chipset circuitry and system memory. Thehost processor may include one or more processor cores and may beconfigured to execute system software. System software may include, forexample, an operating system. System memory may include I/O memorybuffers configured to store one or more data packets that are to betransmitted by, or received by, head-end 102 and/or client device(s) 106a, . . . , 106 n. Chipset circuitry may generally include “North Bridge”circuitry (not shown) to control communication between the processor,I/O circuitry and system memory.

The operating system (OS, not shown) may be configured to manage systemresources and control tasks that are run on, e.g., head-end 102 and/orclient device(s) 106 a, . . . , 106 n. For example, the OS may beimplemented using Microsoft Windows, HP-UX, Linux, or UNIX, althoughother operating systems may be used. In some embodiments, the OS may bereplaced by a virtual machine monitor (or hypervisor) which may providea layer of abstraction for underlying hardware to various operatingsystems (virtual machines) running on one or more processing units. Theoperating system and/or virtual machine may implement one or moreprotocol stacks. A protocol stack may execute one or more programs toprocess packets. An example of a protocol stack is a TCP/IP (TransportControl Protocol/Internet Protocol) protocol stack comprising one ormore programs for handling (e.g., processing or generating) packets totransmit and/or receive over a network.

The system memory may comprise one or more of the following types ofmemory: semiconductor firmware memory, programmable memory, non-volatilememory, read only memory, electrically programmable memory, randomaccess memory, flash memory, magnetic disk memory, and/or optical diskmemory. Either additionally or alternatively system memory may compriseother and/or later-developed types of computer-readable memory.

Embodiments of the operations described herein may be implemented in asystem that includes one or more storage devices having stored thereon,individually or in combination, instructions that when executed by oneor more processors perform the methods. The processor may include, forexample, a processing unit and/or programmable circuitry in the I/Ocircuitry 312, 332 and/or other processing unit or programmablecircuitry. Thus, it is intended that operations according to the methodsdescribed herein may be distributed across a plurality of physicaldevices, such as processing structures at several different physicallocations. The storage device may include any type of tangible,non-transitory storage device, for example, any type of disk includingfloppy disks, optical disks, compact disk read-only memories (CD-ROMs),compact disk rewritables (CD-RWs), and magneto-optical disks,semiconductor devices such as read-only memories (ROMs), random accessmemories (RAMs) such as dynamic and static RAMs, erasable programmableread-only memories (EPROMs), electrically erasable programmableread-only memories (EEPROMs), flash memories, magnetic or optical cards,or any type of storage devices suitable for storing electronicinstructions.

“Circuitry”, as used in any embodiment herein, may comprise, forexample, singly or in any combination, hardwired circuitry, programmablecircuitry, state machine circuitry, and/or firmware that storesinstructions executed by programmable circuitry. “Module”, as usedherein, may comprise, singly or in any combination circuitry and/or codeand/or instructions sets (e.g., software, firmware, etc.).

Communication systems (and methods), consistent with the teachings ofthe present disclosure are configured to provide adaptive channelprediction and/or mitigate interference in OFDM systems. In anembodiment, adaptive channel prediction is configured to predict and/orinterpolate a channel frequency response between channel frequencyresponse estimates that have been determined based on probing symbols.The predicted channel frequency response may then be utilized topre-equalize and/or equalize OFDM symbols. The prediction andinterpolation are configured to accommodate variation in the channelcharacteristics between estimates (i.e., between probing symbols).

In another embodiment, a communication system and method consistent withthe present disclosure are configured to optimize sharing each guardinterval associated with a respective received OFDM symbol. A firstportion of the guard interval may be utilized for accommodatingmicro-reflections and a guard interval remainder may be utilized fortapering using, e.g., a raised cosine window. A duration of the firstportion may be determined based, at least in part, on a modulationscheme, a channel impulse response and signal to noise ratio (SNR)specifications of the modulation scheme. A duration of the guardinterval remainder may then be determined based on the size of the guardinterval and the duration of the first portion. Windowing associated thewith an FFT may then be performed at a receiving device with window edgetapering configured to reduce spectral spread associated with externalinterferers.

Accordingly, the present disclosure provides an example apparatus. Theexample apparatus includes an optimization module configured todetermine a guard interval remainder based, at least in part on acomparison of an allowable microreflection interference level and anactual microreflection interference level; and a windowing moduleconfigured to window an OFDM (orthogonal frequency division multiplexed)symbol utilizing the guard interval remainder. The example apparatus mayfurther include a channel estimator module configured to determine apredicted channel frequency response based, at least in part, on aprobing symbol; and a pre-equalizer module configured to pre-equalizethe OFDM symbol based, at least in part, on the predicted channelfrequency response.

The present disclosure also provides an example method. The examplemethod includes determining, by an optimization module, a guard intervalremainder based, at least in part on a comparison of an allowablemicroreflection interference level and an actual microreflectioninterference level; and windowing, by a windowing module, an OFDM(orthogonal frequency division multiplexed) symbol utilizing the guardinterval remainder. The example method may further include determining,by a channel estimator module, a predicted channel frequency responsebased, at least in part, on a probing symbol; and pre-equalizing, by apre-equalizer module, or equalizing, by an equalizer module, or bothpre-equalizing and equalizing, the OFDM symbol based, at least in part,on the predicted channel frequency response.

The present disclosure also provides an example system that includes oneor more storage devices having stored thereon, individually or incombination, instructions that when executed by one or more processorsresult in the following operations including: determining a guardinterval remainder based, at least in part on a comparison of anallowable microreflection interference level and an actualmicroreflection interference level; and windowing an OFDM (orthogonalfrequency division multiplexed) symbol utilizing the guard intervalremainder. The example system may further include determining apredicted channel frequency response based, at least in part, on aprobing symbol; and pre-equalizing or equalizing or both pre-equalizingand equalizing, the OFDM symbol based, at least in part, on thepredicted channel frequency response.

The present disclosure also provides an example system. The examplesystem includes a transmitting device configured to transmit an OFDM(orthogonal frequency division multiplexed) symbol; and a receivingdevice, comprising: an optimization module configured to determine aguard interval remainder based, at least in part on a comparison of anallowable microreflection interference level and an actualmicroreflection interference level; and a windowing module configured towindow a received OFDM symbol utilizing the guard interval remainder,the received OFDM symbol related to the transmitted OFDM symbol. Theexample transmitting device may further include a channel estimatormodule configured to determine a predicted channel frequency responsebased, at least in part, on a probing symbol. The example transmittingdevice may further include a pre-equalizer module configured topre-equalize the transmitted OFDM symbol based, at least in part, on thepredicted channel frequency response, or the receiving device mayfurther include an equalizer module configured to equalize the receivedOFDM symbol or both.

The terms and expressions which have been employed herein are used asterms of description and not of limitation, and there is no intention,in the use of such terms and expressions, of excluding any equivalentsof the features shown and described (or portions thereof), and it isrecognized that various modifications are possible within the scope ofthe claims. Accordingly, the claims are intended to cover all suchequivalents.

Various features, aspects, and embodiments have been described herein.The features, aspects, and embodiments are susceptible to combinationwith one another as well as to variation and modification, as will beunderstood by those having skill in the art. The present disclosureshould, therefore, be considered to encompass such combinations,variations, and modifications.

What is claimed is:
 1. An apparatus, comprising: an optimization moduleconfigured to determine a guard interval remainder based, at least inpart on a comparison of an allowable microreflection interference leveland an actual microreflection interference level; and a windowing moduleconfigured to window an OFDM (orthogonal frequency division multiplexed)symbol utilizing the guard interval remainder.
 2. The apparatus of claim1, wherein the optimization module is further configured to determinethe allowable microreflection interference level based on a modulationtechnique associated with the OFDM symbol.
 3. The apparatus of claim 1,wherein optimization module is further configured to determine theactual microreflection interference level based, at least in part, on achannel impulse response.
 4. The apparatus of claim 1, furthercomprising: a channel estimator module configured to determine apredicted channel frequency response based, at least in part, on aprobing symbol; and a pre-equalizer module configured to pre-equalizethe OFDM symbol based, at least in part, on the predicted channelfrequency response.
 5. The apparatus of claim 4, wherein thepre-equalizer module is further configured to pre-equalize the probingsymbol.
 6. The apparatus of claim 4, wherein the predicted channelfrequency response is time varying with non-time varying componentsremoved.
 7. A method, comprising: determining, by an optimizationmodule, a guard interval remainder based, at least in part on acomparison of an allowable microreflection interference level and anactual microreflection interference level; and windowing, by a windowingmodule, an OFDM (orthogonal frequency division multiplexed) symbolutilizing the guard interval remainder.
 8. The method of claim 7,further comprising: determining, by the optimization module, theallowable microreflection interference level based on a modulationtechnique associated with the OFDM symbol.
 9. The method of claim 7,further comprising: determining, by the optimization module, the actualmicroreflection interference level based, at least in part, on a channelimpulse response.
 10. The method of claim 7, further comprising:determining, by a channel estimator module, a predicted channelfrequency response based, at least in part, on a probing symbol; andpre-equalizing, by a pre-equalizer module, or equalizing, by anequalizer module, or both pre-equalizing and equalizing, the OFDM symbolbased, at least in part, on the predicted channel frequency response.11. The method of claim 10, further comprising: pre-equalizing, by thepre-equalizer module, the probing symbol.
 12. The method of claim 10,wherein the predicted channel frequency response is time varying withnon-time varying components removed.
 13. A system comprising, one ormore storage devices having stored thereon, individually or incombination, instructions that when executed by one or more processorsresult in the following operations comprising: determining a guardinterval remainder based, at least in part on a comparison of anallowable microreflection interference level and an actualmicroreflection interference level; and windowing an OFDM (orthogonalfrequency division multiplexed) symbol utilizing the guard intervalremainder.
 14. The system of claim 13, wherein the instructions thatwhen executed by one or more processors results in the followingadditional operations comprising: determining the allowablemicroreflection interference level based on a modulation techniqueassociated with the OFDM symbol.
 15. The system of claim 13, wherein theinstructions that when executed by one or more processors results in thefollowing additional operations comprising: determining the actualmicroreflection interference level based, at least in part, on a channelimpulse response.
 16. The system of claim 13, wherein the instructionsthat when executed by one or more processors results in the followingadditional operations comprising: determining a predicted channelfrequency response based, at least in part, on a probing symbol; andpre-equalizing or equalizing or both pre-equalizing and equalizing, theOFDM symbol based, at least in part, on the predicted channel frequencyresponse.
 17. The system of claim 16, wherein the instructions that whenexecuted by one or more processors results in the following additionaloperations comprising: pre-equalizing the probing symbol.
 18. The systemof claim 16, wherein the predicted channel frequency response is timevarying with non-time varying components removed.
 19. A systemcomprising: a transmitting device configured to transmit an OFDM(orthogonal frequency division multiplexed) symbol; and a receivingdevice, comprising: an optimization module configured to determine aguard interval remainder based, at least in part on a comparison of anallowable microreflection interference level and an actualmicroreflection interference level; and a windowing module configured towindow a received OFDM symbol utilizing the guard interval remainder,the received OFDM symbol related to the transmitted OFDM symbol.
 20. Thesystem of claim 19, wherein the optimization module is furtherconfigured to determine the allowable microreflection interference levelbased on a modulation technique associated with the received OFDMsymbol.
 21. The system of claim 19, wherein optimization module isfurther configured to determine the actual microreflection interferencelevel based, at least in part, on a channel impulse response.
 22. Thesystem of claim 19, wherein the transmitting device further comprises: achannel estimator module configured to determine a predicted channelfrequency response based, at least in part, on a probing symbol; and thetransmitting device further comprises a pre-equalizer module configuredto pre-equalize the transmitted OFDM symbol based, at least in part, onthe predicted channel frequency response, or the receiving devicefurther comprises an equalizer module configured to equalize thereceived OFDM symbol or both.
 23. The system of claim 22, wherein thepre-equalizer module is further configured to pre-equalize the probingsymbol.
 24. The system of claim 22, wherein the predicted channelfrequency response is time varying with non-time varying componentsremoved.